1. Field of the Invention
The present invention relates to an anchoring element for fastening a rod of a device for adjusting a human or animal vertebral column on a vertebra, said anchoring element having a retaining means for receiving the rod, a securing element that is attachable on the retaining means and acting against the rod, a fastening element for attachment to the vertebral body and a clamping device which is arranged between the retaining means and the fastening element, including a ring-shaped mount, a bearing having partially the shape of a spherical segment and an intermediate element embedded in the mount and surrounding the bearing, the mount being free to move relative to the bearing when in its released state whilst the mount is clampingly retained on the bearing through the intermediate element when in its clamped state and the mount being rigidly connected to the retaining means and the bearing to the fastening element.
2. Description of the Prior Art
An anchoring element having the features mentioned herein above and implemented as a pedicle screw is known from EP 0 885 598 A2. The fastening element is thereby implemented as a threaded shaft having a bearing formed at the head end thereof. Said bearing is integrally connected with the threaded shaft and is configured in the shape of a sphere in the circumferential direction. The bearing is surrounded by an intermediate element that rests against the bearing in the circumferential direction. The counter surfaces of the intermediate element are curved in such a manner that they register with and fit against the bearing. The intermediate element has three axially oriented slots that are spaced on the circumference so that the segments formed between the slots are radially slidable. The intermediate element is retained in a mount that tapers toward the threaded shaft so that the intermediate element is prevented from falling down. On the mount there is integrally formed a U-shaped retaining means for receiving a connection, compression or distraction rod. In the two retaining crosspieces of the retaining means there is provided an internal buttress thread into which a securing element for fixing the rod, which is implemented as a grub screw, is insertable. The intermediate element is thereby designed in such a manner that its upper region engages into the receiving slot of the retaining means. The term buttress thread is meant to include, besides the buttress metric thread DIN 513, buttress threads having a slightly larger or slightly smaller flank angle, a flank angle of 0° or a negative flank angle as well as buttress threads in accordance with EP 885 598.
Anchoring elements are also known from DE 39 23 996 C2, DE 100 05 385 A1, EP 1 090 595 A2 and WO 98/25534 A1 the pedicle screws of which comprise head ends in the shape of a spherical segment that are clampingly retained in a corresponding retaining means.
To implant said anchoring elements, the threaded shaft is first screwed into the corresponding vertebral body. Then, the rod is placed into the receiving slot and retained by a grub screw. As the grub screw is further tightened, the rod is pressed against the intermediate element so that the mount is displaced relative to the intermediate element. As the mount is configured to be tapered, the segments of the intermediate element are thereby pressed radially together and exert a retaining force onto the bearing. Put another way, in the released condition the mount with the retaining means is free to move with respect to the bearing or to the threaded shaft whilst in the tightened condition, the mount is fixedly and immovably retained on the bearing as a result of the clamping effect. The mount or retaining means may thereby be rotated about the longitudinal axis relative to the bearing and the mount or retaining means can be pivoted transverse the longitudinal axis according to the circular segment of the bearing. This makes it possible to first anchor the pedicle screw in the vertebral body and to then orient the retaining means in the desired manner in order to fix the rod in the optimum position with respect to the vertebral body.
In such-type anchoring elements, the retaining means is frictionally retained on the shaft. When utilizing said anchoring elements on the ventral vertebral column, the forces that are to be transmitted from the rod onto the vertebral body may be very large so that the frictional connection provided is not capable of transmitting these large forces. In such cases, the retaining means comes out of place and the rod is no longer in the desired position.